Small form factor transceiver with externally modulated laser

ABSTRACT

This disclosure is concerned with transceiver modules. In one example, a transceiver module includes a receiver optical subassembly, as well as a transmitter optical subassembly having a header assembly and an externally modulated laser (EML). The header assembly includes a base with first and second sides, as well as a platform attached to the base and having an inside portion near the first side of the base and an outside portion near the second side of the base. The platform of the header assembly further includes a conductive pathway extending through part of the platform. The EML is supported by the platform and electrically communicates with the conductive pathway of the platform. The EML can be passively or actively cooled.

RELATED APPLICATIONS

This application is a divisional, and claims the benefit, of U.S. patentapplication Ser. No. 10/629,253, entitled SMALL FORM FACTOR TRANSCEIVERWITH EXTERNALLY MODULATED LASER, filed Jul. 28, 2003 (the “'253Application). The '253 Application is a continuation-in-part of U.S.patent application Ser. No. 10/625,022, entitled MULTI-LAYER CERAMICFEEDTHROUGH STRUCTURE IN A TRANSMITTER OPTICAL SUBASSEMBLY, and filedJul. 23, 2003, which claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/477,868, filed Jun. 12, 2003. The '253Application is also a continuation-in-part of U.S. patent applicationSer. No. 10/231,395, filed Aug. 29, 2002, which is acontinuation-in-part of U.S. patent application Ser. No. 10/077,067,filed Feb. 14, 2002 and entitled CERAMIC HEADER ASSEMBLY (issued as U.S.Pat. No. 6,586,678). All of the aforementioned patent applications andpatents are incorporated herein in their respective entireties by thisreference.

BACKGROUND OF THE INVENTION

1. Technological Field

This invention is generally concerned with the field of opto-electronicsystems and devices. More specifically, embodiments of the presentinvention relate to an optical transceiver that includes an externallymodulated laser (EML).

2. Related Technology

Fiber-optic and opto-electronics have become important components inmodem networking circuits. Using fiber-optic circuits allows forefficient, accurate and quick transmission of data between variouscomponents in a network system.

As with the design of most any system, there are engineering tradeoffsthat often have to be made when implementing fiber optic systems. Forexample, the size and modularity of components must often be balancedagainst the need for additional space to accommodate heat dissipationand circuit monitoring components. While it is desirable to minimize acomponent's size, some configurations have previously limited thisminimization due to their inherent characteristics. For example,previously many lasers used in fiber-optic systems that have thecharacteristics needed for long-distance transmission and/or densewavelength division multiplexing ([WDM) generated amounts of heat thatcould not be dissipated by some smaller package sizes. Further, smallerpackage sizes have a limited amount of space available for mounting andconnecting additional components such as thermistors, monitorphotodiodes, thermoelectric coolers, or impedance matching circuits.

Regarding smaller package sizes, it is desirable in fiber optic systemsto use modular components so that a system can be created in a compactarea and with as little expensive customization as possible. Forexample, many fiber optic systems are able to use modular transceivermodules. The modular transceiver modules include an input receiveroptical subassembly (ROSA) and an output transmitter optical subassembly(TOSA). The ROSA comprises a photodiode for detecting optical signalsand sensing circuitry for converting the optical signals to digitalsignals compatible with other network components. The TOSA comprises alaser for transmitting optical signals and control circuitry formodulating the laser according to an input digital data signal. The TOSAhas an optical lens for collimating the light signals from the laser ofthe TOSA to an optical fiber. Additionally, the transceiver moduleincludes pluggable receptacles for optically connecting the TOSA and theROSA with other components within a fiber optic network.

The transceiver module often includes an electronic connector forconnection to electrical components of the computer or communicationdevice with which the transceiver module operates (a “host system”). Thedesign of the transceiver, as well as other components within the fiberoptic system, is standards-based, such that components can be connectedwithout significant customization.

One particular pluggable standard that is currently being developed isthe 10-Gigabit Small Form-factor Pluggable (XFP) standard. This standarddefines various characteristics such as size, power consumption,connector configuration, etc. With regards to power consumption, the XFPstandard references three power consumption levels of 1.5 W, 2.5 W and3.5 W. When designing devices to operate within the XFP standard,attention must be given to what components are selected and how they areconfigured so as to not exceed the rated power consumption. Thesedevices are constrained by principles of semiconductor physics to workpreferentially in a certain temperature range. The module powerdissipation and the package size and materials uniquely determine themodule operating temperature for given ambient conditions, such asambient temperature, airflow, etc. The resulting module operatingtemperature determines the types of optical and electronic componentsthat can be successfully operated within the package. One such packageis known as a transistor-outline header, otherwise known as a TO can orTO.

Transistor-outline headers are widely used in the field ofopto-electronics, and may be employed in a variety of applications. Asan example, transistor headers are sometimes used to protect sensitiveelectrical devices, and to electrically connect such devices tocomponents such as printed circuit boards (“PCB”).

With respect to their construction, transistor headers often consist ofa cylindrical metallic base with a number of conductive leads extendingcompletely through, and generally perpendicular to, the base. Withregard to the metallic base, the size of the base is often sized to fitwithin a specific TO standard size and lead configuration, examples ofwhich include a TO-5 or TO-46. The leads are hermetically sealed in thebase to provide mechanical and environmental protection for thecomponents contained in the TO package, and to electrically isolate theconductive leads from the metallic material of the base. Typically, oneof the conductive leads is a ground lead that may be electricallyconnected directly to the base.

Various types of devices are mounted on one side of the base of theheader and connected to the leads. Generally, a cap is used to enclosethe side of the base where such devices are mounted, so as to form achamber that helps prevent contamination or damage to those device(s).The specific characteristics of the cap and header generally relate tothe application and the particular device being mounted on the base ofthe header. By way of example, in applications where an optical deviceis required to be mounted on the header, the cap is at least partiallytransparent so to allow an optical signal generated by the opticaldevice to be transmitted from the TO package. These optical TO packagesare also known as window cans.

Although transistor headers have proven useful, typical configurationsnevertheless pose a variety of unresolved problems. Some of suchproblems relate specifically to the physical configuration anddisposition of the conductive leads in the header base. As an example,various factors combine to compromise the ability to precisely controlthe electrical impedance of the glass/metal feedthrough, that is, thephysical bond between the conductive lead and the header base material.One such factor is that there is a relatively limited number ofavailable choices with respect to the diameter of the conductive leadsthat are to be employed. Further, the range of dielectric values of thesealing glass typically employed in these configurations is relativelysmall. And, with respect to the disposition of the conductive leads, ithas proven relatively difficult in some instances to control theposition of the lead with respect to the through hole in the headerbase.

Yet other problems in the field concern those complex electrical andelectronic devices that require many isolated electrical connections to,function properly. Typically, attributes such as the size and shape ofsuch devices and their subcomponents are sharply constrained by variousform factors, other dimensional requirements, and space limitationswithin the device. Consistent with such form factors, dimensionalrequirements, and space limitations, the diameter of a typical header isrelatively small and, correspondingly, the number of leads that can bedisposed in the base of the header, sometimes referred to as theinput/output (“I/O”) density, is relatively small as well.

Thus, while the diameter of the header base, and thus the I/O density,may be increased to the extent necessary to ensure conformance with theelectrical connection requirements of the associated device, theincrease in base diameter is sharply limited, if not foreclosedcompletely, by the form factors, dimensional requirements, and spacelimitations associated with the device wherein the transistor header isto be employed.

A related problem with many transistor headers concerns the implicationsthat a relatively small number of conductive leads has with respect tothe overall performance of the device and the need to connect additionalcircuitry required by certain types of laser when the transistor headeris used. Semiconductor lasers circuits operate more efficiently when thecircuit driving the semiconductor laser has an impedance that is equalto the impedance of the laser itself There is a special need forimpedance matching and load balancing when circuits are operating atrelatively high frequencies, such as is the case in many semiconductorlaser communication circuits. Mismatched circuits may cause transmissionline reflections and a corresponding inability to maximize the powerdelivered to the semiconductor laser. Additionally, transmission linereflections can cause intensity noise and phase noise that results intransmission penalties in the fiber-optic circuit. Impedance matching isoften accomplished through the use of additional electrical componentssuch as resistors, capacitors, inductors, and transmission lines such asmicrostrips, striplines, or coplanar waveguides. However, suchcomponents cannot be employed unless a sufficient number of conductiveleads are available in the transistor header. Thus, the limited numberof conductive leads present in typical transistor headers has a directnegative effect on the performance of the semiconductor laser or otherdevice.

In connection with the foregoing, another aspect of many transistorheaders that forecloses the use of, for example, components required forimpedance matching, is the relatively limited physical space availableon standard headers. In particular, the relatively small amount of spaceon the base of the header imposes a practical limit on the number ofcomponents that may be mounted thereon. To overcome that limit, some orall of any additional components desired to be used must instead bemounted on the printed circuit board, some distance away from the laseror other device contained within the transistor header. Sucharrangements are not without their shortcomings however, as theperformance of active :devices in the transistor header, such as lasersand integrated circuits, depends to some extent on the physicalproximity of related electrical and electronic components. By minimizingthe distance between the lasers and integrated circuits to theadditional components required for impedance matching, the inherenttransmission line between such components is minimized. As such, placingthe components in close physical proximity reduces reflectivetransmission line losses.

Even when a sufficient number of contacts are available to connectexternal components to the laser for impedance matching, other problemsarise. For example, one of the simplest methods of impedance matching isby shunting a resistive impedance across the laser source wherein the,shunting impedance matches the impedance of the laser. The problem withthis solution is that it adds an additional load to the power supplywhere the additional load is the shunt resistor and thus wastes powerand generates heat.

In one example, suppose that a laser has a 25 ohm load impedance and alaser driver has a 12.5 ohm source impedance. To match the laserimpedance, a 25 ohm resistor is shunted across the laser. This resultsin a 12.5 ohm load for the laser driver that, while impedance matched,requires more power to drive than if the laser driver only needed todrive a 25 ohm load. One way to eliminate the need for externalcomponents is to create an appropriately designed transmission line thattransmits the laser signal from the laser driver to the laser itself,with proper characteristic impedance to match the laser and the laserdriver. In this way, the laser driver efficiently supplies power to the25 ohm load while minimizing harmful reflections. Such transmissionlines are often appropriately sized microstrips, striplines, or coplanarwaveguides, etc., formed on a printed circuit board using thecharacteristics of the conductive materials on the circuit board and thesubstrate on which the conductive materials are placed. As such, whereastransistor headers do not have internal printed circuit boardsavailable, such matching transmissions lines cannot be constructed.

In addition to the need for matching circuits, there is also often aneed for other additional circuitry. For example, an externallymodulated laser (EML) comprises a laser and a semiconductor modulator.Examples of lasers that can be used with EMLs include a distributedfeedback (DFB) laser or a distributed Bragg reflector (DBR) laser.Examples of modulators include an electroabsorptive modulator, in whichthe modulator absorbs light depending on a control voltage, or variousinterferometric modulators, such as the Mach-Zehnder modulator, oftenmade with lithium niobate. An eternally modulated laser having anelectroabsorptive modulator can be referred to as an EA EML(electroabsorbtive externally modulated laser). The integrated modulatorhas additional connections that require control signals from devicesexternal to the transistor header that are normally not required when alaser without the integrated modulator is included. As such, withoutadditional connections, lasers, such as EMLs, cannot be implemented incurrent transistor header designs.

The problems associated with various typical transistor headers are not,however, limited solely to geometric considerations and limitations. Yetother problems relate to the heat generated by components within, andexternal to, the transistor header. Specifically, transistor headers andtheir associated subcomponents may generate significant heat duringoperation. It is generally necessary to reliably and efficiently removesuch heat to optimize performance and extend the useful life of thedevice.

However, transistor headers are often composed primarily of materials,Kovare for example, that are not particularly good thermal conductors,but are instead selected for their properties of minimum thermalexpansion and contraction, to match glass-metal seals and guaranteehermeticity. Such poor thermal conductivity does little to alleviateheat buildup problems in the transistor header components and may, infact, exacerbate such problems. Various cooling techniques and deviceshave been employed in an effort to address this problem, but with onlylimited success. Such cooling problems have limited the types of lasersthat may be used in transistor header applications. Particularly, suchcooling problems have presented significant barriers to using lasersthat are adapted for long-range fiber-optic communications such asexternally modulated lasers (EMLs) that generate significant amounts ofheat.

One drawback of using an EML is the heat that is generated by such alaser. Typically, most EMLs are-operated between 25° C. to 30° C. Assuch, external cooling has commonly been required to pump heat away fromthe EML to maintain the laser at an appropriate operating temperature.The need for cooling components has previously imposed a limitation onthe size of packages into which an EML is integrated. Further, becauseof the need for active cooling, the power consumption of a deviceintegrating an EML is often greater than that allowed by many of thesmaller package size standards such as XFP. Previously, EMLs have notbeen effectively integrated into smaller packages because of thesecooling requirements. Additionally, in order to keep a laser'swavelength stable to enable such applications as DWDM, the temperaturemust be finely controlled to be fixed regardless of varying ambienttemperatures and conditions. One of the best methods to accomplish thistemperature control is to have precise control of the same cooler thatis used to keep the laser at an appropriate operating temperature.

Solid state heat exchangers may be used to remove some heat fromtransistor header components. However, the effectiveness of such heatexchangers is typically compromised because, due to variables such astheir configuration and/or physical location relative to the primarycomponent(s) to be cooled, such heat exchangers frequently experience apassive heat load that is imposed by secondary components or transistorheader structures not generally intended to be cooled by the heatexchanger. The imposition on the heat exchanger of such passive heatloads thus decreases the amount of heat the heat exchanger caneffectively remove from the primary component that is desired to becooled, thereby compromising the performance of the primary component.

As suggested above, the physical location of the heat exchanger or othercooling device has various implications with respect to the performanceof the components employed present in the transistor header. Oneparticular problem in the context of thermoelectric cooler (“TEC”) typeheat exchangers arises because TECs have hot and cold junctions. Thecold junction, in particular, can cause condensation if the TEC islocated in a sufficiently humid environment. Such condensation maymaterially impair the operation of components in the transistor header,and elsewhere.

Solid state coolers, such as TECs, are intrinsically very inefficientdevices. State-of-the-art coolers have efficiencies measured in singleor low double digits. Thus, the power consumption becomes astronomicalwhen an attempt is made to cool lasers in packages that have significantthermal leaks. This process requires large amounts of power, which isinappropriate for small devices because it causes large temperaturerises and because it is not permitted under standards, such as the XFPstandard.

Another concern with respect to heat exchangers is that the dimensionsof typical transistor headers are, as noted earlier, constrained byvarious factors. Thus, while the passive heat load placed on a heatexchanger could be at least partly offset through the use of arelatively larger heat exchanger, the diametric and other constraintsimposed on transistor headers by form factor requirements and otherconsiderations place practical limits on the maximum size of the heatexchanger.

Finally, even if a relatively large heat exchanger could be employed inan attempt to offset the effects of passive heat loads, large heatexchangers present problems in cases where the heat exchanger, such as aTEC, is used to modify the performance of transistor header componentssuch as lasers. For example, by virtue of their relatively large thermalmass or load, such heat exchangers are not well suited to implementingthe rapid changes in laser performance that are required in manyapplications, because such large heat exchangers cannot transfer theheat rapidly enough. Moreover, the performance of the laser or othercomponent may be further compromised if the heat exchanger is locatedrelatively far away from the laser because the thermal resistance isproportional to the distance between the component and the heatexchanger.

BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION

In general, embodiments of the invention are concerned with a transistorheader including various features directed to enhancing the reliabilityand performance of various electronic devices, such as lasers, includedin the transistor header.

In one exemplary embodiment of the invention, a transceiver module isprovided that substantially complies with the XFP MSA. This exemplarytransceiver module includes a receiver optical subassembly, as well as atransmitter optical subassembly having a header assembly and anexternally modulated laser (EML). The header assembly includes a basewith first and second sides, as well as a platform attached to the baseand having an inside portion near the first side of the base and anoutside portion near the second side of the base. The platform of theheader assembly further includes a conductive pathway extending throughpart of the platform. The EML is supported by the platform andelectrically communicates with the conductive pathway of the platform.The EML can be passively or actively cooled.

These, and other, aspects of exemplary embodiments of the presentinvention will become more fully apparent from the following descriptionand appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand features of the invention are obtained, a more particulardescription of the invention briefly described above will be rendered byreference to specific embodiments thereof which are illustrated in theappended drawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be consideredlimiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1A is a perspective view illustrating various aspects of the deviceside of an exemplary embodiment of a header assembly;

FIG. 1B is a perspective view illustrating various aspects of theconnector side of an exemplary embodiment of a header assembly;

FIG. 2A is a perspective view illustrating various aspects of the deviceside of an alternative embodiment of a header assembly;

FIG. 2B is a perspective view illustrating various aspects of theconnector side of an alternative embodiment of a header assembly;

FIG. 3A is a perspective view illustrating various aspects of the deviceside of another alternative embodiment of a header assembly;

FIG. 3B is a perspective view illustrating various aspects of theconnector side of another alternative embodiment of a header assembly;

FIG. 4A is a top perspective view of an exemplary embodiment of a headerincluding active devices mounted on a TEC disposed within a hermeticchamber;

FIG. 4B is a bottom perspective view of the exemplary embodimentillustrated in FIG. 4A;

FIG. 4C is a cross-section view illustrating various aspects of theexemplary embodiment presented in FIGS. 4A and 4B;

FIG. 4D is a cross-section view taken along line 4D-4D of FIG. 4C andillustrates various aspects of an exemplary arrangement of a TEC in aheader assembly;

FIG. 4E is a side view illustrating aspects of an exemplary electricalconnection scheme for the header assembly and a printed circuit board;

FIG. 4F illustrates various aspects of an alternative platform/TECconfiguration where the TEC is located outside the hermetic chamber;

FIG. 5 is a block diagram of an exemplary laser control system;

FIG. 6 is a perspective view of an exemplary transmitter opticalsubassembly with a transistor header assembly and an EML, as well asoptics, such as a lens, isolator, and a receptacle for an optical cablesuch as an LC cable; and

FIG. 7 is a perspective view of a small form factor XFP opticaltransceiver module having an EML.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Reference will now be made to figures wherein like structures will beprovided with like reference designations. It is to be understood thatthe drawings are diagrammatic and schematic representations of variousembodiments of the claimed invention, and are not to be construed aslimiting the scope of the present invention in any way, nor are thedrawings necessarily drawn to scale.

According to the invention, EML lasers are incorporated into a headerassembly that, permits the EML lasers to be used with small form factoroptical transceiver modules. The components of the header assemblies,including the thermoelectric coolers, are disclosed herein. In addition,the use of EML lasers in such header assemblies is disclosed.

An EML is generally constructed from a laser, such as a distributedfeedback (DFB) laser or a distributed Bragg reflector (DBR) laser and amodulator. This construction allows the EML to have a narrow line widthor channel spacing. Further, the EML has low chirp or frequency driftcompared to a directly mounted laser (DML). In addition, EMLs provide arelatively high extinction-ratio (ER) signal, which is easier to detectat long distances. The ER is the ratio power in a logic “1” compared toa logic “0”, and is thus a measure of contrast or detectability. EMLsare particularly useful in long-distance applications because of thesecharacteristics. Particularly, narrow line width and low chirp allowsfor a high bit rate as these characteristics cause the transmitted beamto be resistant to the dispersion that causes transmission errors inadjacent transitions. For example, in a 1550 nanometer application, thedispersion in a fiber-optic fiber is high. However using an EML in a 10Gb/s system, transmissions of 40 to 80 km can be achieved, compared withonly about 10 km with a DFB DML. In a 2.5 Gb/s system, 160 kmtransmissions are possible, compared to only about 40 km that can beachieved using a DFB DML.

In a multiple channel system, narrow line width and low chirp allows foradjacent channels to be propagated near one another while channelcrossover is minimized. A stabilized wavelength characterized by narrowline width and low chirp is important so that the fiber-optic channelcan operate along a predefined ITU wavelength channel. Typically an ITUwavelength for a dense wavelength division multiplexing (WDM) systemoperates such that the channel spacings are 100 GHz, 50 GHz or 25 GHz.Narrow channel spacings require precise control of the wavelength andlow chirp. In addition, a high extinction ratio is important to achievelong distance transmissions. As noted above, EMLs exhibit suchcharacteristics and are suitable for use in DWDM and long-haul systems.According to the present invention, an EML can be used in an XFP opticaltransceiver module, providing the benefits of both EMLs and themodularity, form factor, low power, and the other advantages ofcompliance with the XFP standard.

At the time of the filing of this patent application, the XFP standardis the XFP Revision 2.0 Public Draft for Comments, promulgated by the 10Gigabit Small Form Factor Pluggable (XFP) Multi Source Agreement (MSA)Group. This XFP Revision 2.0 document is incorporated herein byreference. In addition, a newer XFP Revision 3.0 is being developed, andincludes similar requirements. As used herein, the terms “XFP standard”and “XFP Multi Source Agreement” refer to the Revision 2.0 Public Draftfor Comments. These terms also refer to any subsequent drafts, such asXFP Revision 3.0 or final agreements to the extent that any suchsubsequent drafts or final agreements are compatible with Revision 2.0.

FIGS. 1A-3B are used to describe herein details associated with a headerassembly that has a feedthrough assembly for providing electricalconnection within a hermetically sealed chamber. FIGS. 4A-5 are referredto herein to describe header assemblies with integrated thermoelectriccoolers that can be used to dissipate heat from active components withinthe header assemblies, including EML lasers or other types of lasers.FIG. 5 illustrates a TOSA with an EML laser housed within a headerassembly with an integrated thermoelectric cooler constructed accordingto the invention.

1. Header Assemblies

Reference is first made to FIGS. 1A and 1B together, which illustrateperspective views of one presently preferred embodiment of a headerassembly, designated generally at 200. In the illustrated example, theheader assembly 200 includes a substantially cylindrical metallic base10. The base 10 includes two flanges 90 for releasably securing theheader 200 to a receptacle (not shown) on a higher level opto-mechanicalassembly. The base can be formed of Alloy 42, which is an iron nickelalloy, as well as cold-rolled steel, Vacon VCF-25 Alloy, or Kovar. Thebase 10 also includes a ceramic platform 70 extending perpendicularlythrough the base as shown. The ceramic platform is hermetically sealedto the base to provide mechanical and environmental protection for thecomponents contained in the TO package. Ceramic materials may includealumina (Al₂O₃) or aluminum nitride (AlN).

The hermetic seal between the base 10 and the platform 70 is created byelectrically insulating glass-to-metal seals. Alternatively, theplatform 70 may incorporate two additional ceramic outer layers toelectrically isolate the outermost conductors. In this second case, ametal braze or solder can be used to hermetically seal the platform 70to the metal base. This solution overcomes the principal shortcomings ofglasses, namely their low strength, brittleness, and low thermalconductivity.

The platform 70 is structured to house multiple electrical components 50and 100, and active devices 60 on either side of the base. In theillustrated embodiment, the active device 60 comprises a semiconductorlaser, and the components 50 and 100 may include resistors, capacitors,and inductors that are used to balance the driving impedance of thelaser with the component impedance. As discussed in more detail below,impedance matching circuits may also be created by etching electricaltraces that have various capacitive, inductive or resistive properties,on platform 70. In addition to matching, components may have peripheralfunctions such as measuring temperature, sensing laser optical power orwavelength, etc. As it is important for a semiconductor laser to beprecisely positioned perpendicularly to the base 10, platform 70 is,therefore, precisely positioned perpendicularly with respect to the base10.

Where active device 60 comprises a semiconductor laser, a smalldeviation in the position of active device 60, in relation to base 10can cause a large deviation in the direction of the emitted laser beam.Accurate perpendicularity between the platform and the base can beachieved by incorporating a vertical pedestal feature in the basematerial, as shown on FIG. 1A. The vertical pedestal houses thephotodiode 30 in the embodiment shown in FIG. 1A. Such feature can bemachined, stamped, or metal injection molded directly with the base thusproviding a stable and geometrically accurate surface for mating withthe platform.

The platform 70 further includes multiple electrically isolatedconductive pathways 110 extending throughout the platform 70 andconsequently through the base 10. The conductive pathways 110 providethe electrical connections necessary between electrical devices orcomponents located throughout the platform 70. The conductive pathways110 form a connector on that side of the base that does not include thesemiconductor laser 60, also referred to herein as the “connector side”of the base. Note in connection with the foregoing that the side of thebase where the active device 60 is located may in some instances bereferred to herein as the “device side” of the base.

The connector formed by the conductive pathways 110 is used toelectrically connect the header assembly 200 to a second electricalsubassembly, such as a printed circuit board, either directly (forexample, by solder connection) or indirectly by an intermediary devicesuch as a flexible printed circuit. The semiconductor laser 60 iselectrically connected to the electrical components 50 and 100 via theconductive pathways 110.

The platform 70 may also comprise multiple layers wherein each layer mayhave a conductive layer with various conductive pathways 110. In thisway numerous conductive pathways 110 may be constructed for use withvarious components disposed on the platform 70. Generally, the layersare electrically isolated from one another, however various conductivepathways 110 on different layers may be connected by a via such as iscommonly known in printed circuit board arts.

Further, the conductive pathways 110 can be shaped and placed such thatthey have controlled capacitive, inductive, or resistive effects tocreate waveguides such as a microstrip or stripline (cpw, etc.). Forexample, knowing certain characteristics about the materials used inmaking the conductive pathways 1 10 and the materials of the variouslayers of the platform 70, passive electrical devices can be constructedby appropriately configuring the conductive pathways 110. In this way, atransmission line with known characteristics can be created for use withactive devices 60 attached to the platform 70. As noted above, bymatching the characteristics of the transmission line connected toactive devices 60 with the active devices' 60 load impedance, electricalreflections that cause transmission errors and lower power output can bereduced or in many cases eliminated.

By constructing a transmission line that matches active device 60impedance on the platform 70 from the conductive traces 110, the need toadd additional discrete matching components is eliminated oftenresulting in better overall circuit performance. In fact, previously dueto the lack of adequate matching circuits, applications involvingtransistor headers have been limited to 10 Gb/s. With the improvementsof using a transmission line constructed on the platform 70,applications up to 40 Gb/s or more can be implemented.

While the preceding description has discussed active devices 60 in termsof lasers, it should be noted that the transmission lines may also beformed such that a matching circuit for other semiconductor devices isconstructed. For example, the transmission lines may be used to connectdirectly to a laser, such as in the case of DFB lasers. Alternately, thetransmission lines may be used to connect to an EA modulator, forexample, such as in the case of EMLs that incorporate a DFB laser and anEA modulator. As discussed herein, the impedance values of the impedancematching transmission lines depend on the load impedance of the activedevices attached to the platform 70.

External components, while still useful, are not ideal for impedancematching because they often represent an additional load that must bedriven by the power supply driving the electronic component, such aswhen resistors are used to match the active device 60 load; impedance.Additionally, although the external components may be placed reasonablyclose to the active devices 60, there is always some small distancebetween the external components and the active devices 60 that acts asan unmatched transmission line.

The use of advanced ceramic materials, examples of which includealuminum nitride and beryllia, allows the header assembly 200 to achievesubstantially lower thermal resistances between the devices inside thepackage and the outside world where heat is ultimately transferred. Asdiscussed in further detail below in the context of an alternativeembodiment of the invention, a cooling device, such as a thermoelectriccooler (“TEC”), a heat pipe or a metal heat spreader, can be mounteddirectly on the platform, thereby providing for a very short thermalpath between the temperature sensitive devices on the platform and aheat sink located outside the header assembly.

As is further shown in FIGS. 1A and 1B, the header assembly 200additionally includes two conductive leads 40 extending through and outboth sides of the base 10. The conductive leads 40 are hermeticallysealed to the base 10 to provide mechanical and environmental protectionfor the components contained in the TO package between the conductiveleads 40 and the base 10. The hermetic seal between the conductive leads40 and the base 10 is created, for example, by glass or other comparablehermetic insulating materials that are known in the art. The conductiveleads 40 can also be used to electrically connect devices and/orcomponents located on opposite sides of the base.

In the illustrated embodiment at least, the conductive leads 40 extendout from the side of the base 10 that does not contain the semiconductorlaser 60, in a manner that allows for the electrical connection of theheader assembly 200 with a specific header receptacle located on, forexample, a printed circuit board. It is important to note thatconductive pathways 110 and conductive leads 40 perform the samefunction and that the number of potential conductive pathways 110 is fargreater than the potential number of conductive leads 40. Alternativeembodiments can incorporate even more conductive pathways 110 than shownin the illustrated embodiment.

The platform 70 further includes steps and recessed areas that permitmounting devices with various thicknesses flush with the metal pads onthe ceramic. This allows the use of the shortest electricalinterconnects, wire bonds for example, having improved electricalperformance and characteristics. This also provides optical benefits by,for example, aligning the active region of a laser mounted on theplatform with the optical axis of the package.

The photodiode 30 is used to detect the signal strength of thesemiconductor laser 60 and relay this information back to controlcircuitry of the semiconductor laser 60. In the illustrated embodiment,the photodiode can be directly connected to the conductive leads 40.Alternatively, the photodiode can be mounted directly onto the sameplatform as the laser, in a recessed position with respect to the lightemitting area. This recessed position allows the photodiode to capture afraction of the light emitted by the laser, thus allowing the photodiodeto perform the same monitoring function.

This configuration of the monitoring photodiode allows for eliminatingthe need of conductive leads 40, and lends itself to simplifiedelectrical connections, such as wire bonds, to the conductive pathways110 of the platform 70. In an alternative embodiment, the photodiodelight gathering can be increased by positioning an optical element onthe base for focusing or redirecting light, such as a mirror, or bydirectly shaping and/or coating the base metal to focus additional lightonto the photodiode.

As is further shown in FIG. 1A, the base 10 includes a protrudingportion 45 that is configured to releasably position or locate a cap(not shown) over one side of the base 10. A cap can be placed over theside of the base 10 containing the semiconductor laser 60 for thepurpose of protecting the semiconductor laser 60 from potentiallydestructive particles. A transparent cap is preferable for theillustrated embodiment so as to allow the laser light to escape theregion between the cap and the base 10.

Reference is next made to FIGS. 2A and 2B, which illustrate perspectiveviews of an alternative embodiment of a header assembly, designatedgenerally at 300. This alternative embodiment shows an optical receiver360 mounted horizontally on the platform 370 perpendicularly bisectingthe base 310 of the header assembly 300. The optical receiver can be aphotodetector or any other device capable of receiving optical signals.The optical receiver 360 is mounted flat on the platform 370 and detectslight signals through the side facing away from the base 310. This typeof optical receiver is sometimes referred to as an “edge detecting”detector. The base 310 and platform 370 are described in more detailwith reference to FIGS. 1A and 1B. The platform 370 contains electricalcomponents 350, 400 on either side of the base for operating the opticalreceiver 360. The platform 370 also includes conductive pathways 410 forelectrically connecting devices or components on either side of the base310. This embodiment of a header assembly does not contain conductiveleads and therefore all electrical connections are made via theconductive pathways 410.

Reference is next made to FIGS. 3A and 3B, which illustrate perspectiveviews of yet another alternative embodiment of a header assembly,designated generally at 500. This alternative embodiment also shows anoptical receiver 530 mounted vertically on the base 510. The opticalreceiver can be a photodetector or any other device capable of receivingoptical signals. This is an optical receiver 530 which detects lightsignals from the top of the device. The base 510 and platform 570 aredescribed in more detail with reference to FIGS. 1A and 1B. The platform570 contains electrical components 550, 600 on either side of the basefor operating the optical receiver 530. The platform 570 also includesconductive pathways 510 for electrically connecting devices orcomponents on either side of the base 510. This embodiment of a headerassembly does not contain conductive leads and therefore all electricalconnections are made via the conductive pathways 410.

In other embodiments of the invention, the optical receiver 360 oroptical receiver 530 is an avalanche photodiode (APD). Generally, APDsrepresent a good choice for an optical receiver because they have goodnoise and gain characteristics. Specifically, the wide gain bandwidthproduct of APDs allows for more versatility in design such that noisecan be reduced and transmission distances increased. Unlike thetransmitter designs disclosed herein, these receivers often includeactive semiconductor integrated circuits mounted next to the receiverpin diode or APD, generally in the form of a transimpedance amplifier(TIA) or a TIA with a limiting amplifier (TIALA).

2. Thermoelectric Coolers Used with Header Assemblies

Directing attention now to FIGS. 4A through 4D, various aspects of analternative embodiment of a header assembly, generally designated at700, are illustrated. The embodiment of the header assembly illustratedin FIGS. 4A through 4D is similar in many regards to one or more of theembodiments of the header assembly illustrated in FIGS. 1A through 3B.Accordingly, the discussions of FIGS. 4A through 4D will focus primarilyon certain selected aspects of the header assembly 700 illustratedthere. Note that in one embodiment of the invention, header assembly 700comprises a transistor header. However, header assembly 700 is notlimited solely to that exemplary embodiment.

As indicated in FIGS. 4A through 4D, header assembly 700 generallyincludes a base 702 through which a platform 800 passes. The platform800 is configured to receive a cooling device 900 upon which variousdevices and circuitry are mounted. Note that while it may be referred toherein as a “cooling” device 900, the cooling device 900 may, dependingupon its type and the application where it is employed, serves both toheat and/or cool various components and devices. Finally, a cap 704mounted to, and cooperating with, base 702, serves to define a hermeticchamber 706 which encloses cooling device 900 and the mounted devicesand circuitry.

As discussed in further detail below, a variety of means may be employedto perform the functions disclosed herein, of a cooling device. Thus,the embodiments of the cooling device disclosed and discussed herein arebut exemplary structures that function as a means for transferring heat.Accordingly, it should be understood that such structural configurationsare presented herein solely by way of example and should not beconstrued as limiting the scope of the present invention in any way.Rather, any other structure or combination of structures effective inimplementing the functionality disclosed herein may likewise beemployed.

With continuing attention to FIGS. 4A and 4B, and directing attentionalso to FIGS. 4C and 4D, further details are provided concerning variousaspects of platform 800. In the illustrated embodiment, platform 800 isdisposed substantially perpendicularly with respect to base 702. Inparticular, base 702 includes a device side 702A and a connector side702B, and platform 800 passes completely through base 702, so that aninside portion 801A of platform 800 is disposed on device side 702A ofbase 700 and outside portion 801B of platform 800 is disposed onconnector side 702B of base 702. However, this arrangement of platform800 is exemplary only, and various other arrangements of platform 800may alternatively be employed consistent with the requirements of aparticular application.

In the illustrated embodiment, platform 800 includes a first feedthrough802 having a multi-layer construction that includes one or more layers804 of conductive pathways 806 (see FIG. 4A). In general, conductivepathways 806 permit electrical communication among the variouscomponents and devices (removed for clarity) disposed on platform 800,while also permitting such components and devices to electricallycommunicate with other components and devices that are not a part ofplatform 800. Moreover, conductive pathways 806 cooperate to form aconnector 810 situated on the outside portion 801B of platform 800, onthe connector side 702B of base 700. In general, connector 810facilitates electrical communication between header assembly 700 andother components and devices such as, but not limited to, printedcircuit boards (see FIG. 4E). In one embodiment, connector 810 comprisesan edge connector, but any other form of connector may alternatively beused, consistent with the requirements of a particular application. Asdiscussed in further detail below, first feedthrough 802 may includecutouts 811 or other geometric features which permit direct access to,and electrical connection with, one or more conductive pathways 806disposed on an inner layer of first feedthrough 802.

In addition to the first feedthrough 802, platform 800 further includesa second feedthrough 812 to which the first feedthrough 802 is attached.Note that in the exemplary illustrated embodiment, first feedthrough810, with the exception of conductive pathways 806, often is formed froma ceramic material that is generally resistant to heat conduction.However, other ceramic materials, such as AlN, are conductive of heatand can be used to assist in the transfer of heat out of the package.Second feedthrough 812 in the illustrated embodiment comprises amaterial that is generally useful as a heat conductor, such as a metal.Copper and copper alloys, such as CuW, are examples of metals that aresuitable in some applications. Thus, platform 800 is generallyconfigured to combine heat conductive elements with non-heat conductiveelements so as to produce a desired effect or result concerning thedevice wherein platform 800 is employed.

In connection with the foregoing, it should be noted further thatceramics and metals are exemplary materials only and any other materialor combination thereof that will facilitate implementation of thefunctionality disclosed herein may alternatively be employed. Moreover,other embodiments of the invention may employ different arrangements andnumbers of, for example, conductive and non-conductive feedthroughs, orfeedthroughs having other desirable characteristics. Accordingly, theillustrated embodiments are exemplary only and should not be construedto limit the scope of the invention in any way.

With respect to their configurations, the geometry of both firstfeedthrough 802 and second feedthrough 812 may generally be configuredas necessary to suit the requirements of a particular application ordevice. In the exemplary embodiment illustrated in FIGS. 4A through 4D,second feedthrough 812 incorporates a step 812A feature which serves to,among other things, provide support for cooling device 900 and, asdiscussed in further detail below, to ensure that devices mounted tocooling device 900 are situated at a desirable location and orientation.As further indicated in FIG. 4D, for example, second feedthrough 812defines a semi-cylindrical bottom that generally conforms to the shapeof cap 704 and contributes to the stability of cooling device 900, aswell as providing a relatively large conductive mass that aids in heatconduction to and/or from, as applicable, cooling device 900 and otherdevices.

As suggested earlier, platform 800 also serves to provide support tocooling device 900. Directing renewed attention now to FIGS. 4A through4D, details are provided concerning various aspects of cooling device900. In particular, a cooling device 900 is provided that is mounted isdirectly to platform 800. In an exemplary embodiment, cooling device 900comprises a thermoelectric cooler (“TEC”) that relies for its operationand usefulness on the Peltier effect wherein electrical power suppliedto the TEC may, according to the requirements of a particularapplication, cause selected portions of the TEC to generate heat and/orprovide a cooling effect. Exemplary construction materials for the TECmay include, but are not limited to, bismuth telluride (Bi₂Te₃), andother such materials designed to maximize the thermo-electric effect.These materials are selected to have minimum thermal conductivity, sinceit is directly parasitic to the cooling/heating effect (one side getscold, the other hot, and the device itself is a direct short). Theplatform 800 is highly thermally conductive, and can be formed from Cuor CuW.

Note that the TEC represents an exemplary configuration only, andvarious other types of cooling devices may alternatively be employed asrequired to suit the dictates of a particular application. By way ofexample, where active temperature control of one or more electronicdevices 1000, aspects of which are discussed in more detail below, isnot required, the TEC may be replaced with a thermally conductivespacer, laser control circuitry, laser power supply circuitry or othersimilar devices. Furthermore, a combination of devices may be placedinto transistor header in the location showing the TEC in FIGS. 4 a-4 d.

In addition to providing heating and/or cooling functionality, coolingdevice 900 also includes a submount 902 that supports various electronicdevices 1000 such as, but not limited to, resistors, capacitors, andinductors, as well as optical devices such as mirrors, lasers, andoptical receivers. Thus, cooling device 900 is directly thermallycoupled to electronic devices 1000.

In one exemplary embodiment, the electronic devices 1000 include a laser1002, such as a semiconductor laser, or other optical signal source.With regard to devices such as laser 1002, at least, cooling device 900is positioned and configured to ensure that laser 1002 is maintained ina desired position and orientation. By way of example, in someembodiments of the invention, cooling device 900 is positioned so thatan emitting surface of laser 102 is positioned at, and aligned with, alongitudinal axis A-A of header assembly 700 (FIG. 4C).

Note that although reference is made herein to the use of a laser 1002in conjunction with cooling device 900, it should be understood thatembodiments employing laser 1002 are exemplary only and that additionalor alternative devices may likewise be employed. Accordingly, the scopeof the invention should not be construed to be limited solely to lasersand laser applications.

In at least some of those embodiments where a laser 1002 is employed, aphotodiode 1004 and thermistor 1006 are also mounted on, or proximateto, submount 902 of cooling device 900. In general, photodiode 1004 isoptically coupled with laser 1002 such that photodiode 1004 receives atleast a portion of the light emitted by laser 1002, and thereby aids ingathering light intensity data concerning laser 1002 emissions. Further,thermistor 1006 is thermally coupled with laser 1004, thus permittingthe gathering of data concerning the temperature of laser 1002. Theremay also be a wavelength locking circuit having two separate photodiodeswith different wavelength-sensitive responses, which is known as awavelocker.

In some embodiments, photodiode 1004 comprises a 45 degree monitorphotodiode. The use of this type of diode permits the relatedcomponents, such as laser 1002 and thermistor 1006 for example, to bemounted and wirebonded on the same surface. Typically, the 45 degreemonitor diode is arranged so that light emitted from the back of laser1002 is refracted on an inclined surface of the monitor diode andcaptured on a top sensitive surface of the monitor diode. In this way,the monitor diode is able to sense the intensity of the optical signalemitted by the laser.

Note that in those embodiments where a laser 1002 is employed, cap 704includes an optically transparent portion, or window, 704A through whichlight signals generated by the laser 1002 are emitted. Similarly, in theevent electronic device 1000 comprises other optical devices, such as anoptical receiver, cap 704 would likewise include a window 704A so as topermit reception, by the optical receiver, of light signals. Assuggested by the foregoing, the construction and configuration of cap704 may generally be selected as required to suit the parameters of aparticular application.

In view of the foregoing general discussion concerning variouselectronic devices 1000 that may be employed in conjunction with coolingdevice 900, further attention is directed now to certain aspects of therelation between such electronic devices 1000 and cooling device 900. Ingeneral, cooling device 900 may be employed to remove heat from, or addheat to, one or more of the electronic devices 1000, such as laser 1002,to achieve a desired effect. As discussed in further detail herein, thecapability to add and remove heat, as necessary, from a device such aslaser 1002, may be employed to control the performance of laser 1002,such as wavelength stability for DWDM applications

In an exemplary embodiment, the heating and cooling, as applicable, ofelectronic devices 1000 is achieved with a cooling device 900 thatcomprises a TEC. Various aspects of the arrangement and disposition ofelectronic devices 1000, as well as cooling device 900, serve to enhancethese ends. By way of example, because electronic devices 1000 aremounted directly to cooling device 900 results in a relatively shortthermal path between electronic devices 1000 and cooling device 900.Generally, such a relatively shorter thermal path between componentstranslates to a corresponding increase in the efficiency with which heatmay be transferred between those components. Such a result isparticularly useful where devices whose operation and performance ishighly sensitive to heat and temperature changes, such as lasers, areconcerned. Moreover, a relatively short thermal path also permits thetransfer of heat to be implemented relatively more quickly than wouldotherwise be the case. Because heat transfer is implemented relativelyquickly, this exemplary arrangement can be used to effectively andreliably maintain the temperature of laser 1002 or other devices.

Another aspect of at least some embodiments relates to the location ofcooling device 900 relative, not just to electronic devices 1000, but toother components, devices, and structures of header assembly 700. Inparticular, because cooling device 900 is located so that the potentialfor heat transmission, whether radiative, conductive, or convective,from other components, devices, and structures of header assembly 700 tocooling device 900 is relatively limited, the passive heat load imposedon cooling device 900 by such other components and, structures isrelatively small. Note that, as contemplated herein, the “passive” heatload generally refers to heat transferred to cooling device 900 bystructures and devices other than those upon which cooling device 900 isprimarily intended to exert a heating and/or cooling effect. Thus, inthis exemplary embodiment, “passive” heat loads refers to all heat loadsimposed on cooling device 900 except for those heat loads imposed byelectronic devices 1000.

The relative reduction in heat load experienced by cooling device 900 asa consequence of its location has a variety of implications. Forexample, the reduced heat load means that a relatively smaller coolingdevice 900 may be employed than would otherwise be the case. This is adesirable result, particularly in applications such as header assemblieswhere space may be limited. As another example, a relatively smallercooling device 900, at least where cooling device 900 comprises a TEC,translates to a relative decrease in the amount of power required tooperate cooling device 900. This effect is quite significant, since TECsare very inefficient. The power to effectively cool is much more thanthe load, so any reduction in load has a multiplicative benefit. Anotherconsideration relating to the location of cooling device 900 concernsthe performance of laser 1002 and the other electronic components 1000disposed in hermetic chamber 706. In particular, the placement ofcooling devices 900, such as TECs that include a “cold” connection, inhermetic chamber 706 substantially forecloses the occurrence ofcondensation, and the resulting damage to other components and devicesof header assembly 700, caused by the cold connection, that mightotherwise result if cooling device 900 were located outside hermeticchamber 706.

In addition to the heat transfer effects that may be achieved by way ofthe location of cooling device 900, and the relatively short thermalpath that is defined between cooling device 900 and the electronicdevices 1000 mounted to submount 902 of cooling device 900, yet otherheat transfer effects may be realized by way of various modifications tothe geometry of cooling device 900. In connection with the foregoing, itis generally the case that by increasing the size of cooling device 900,a relative increase in the capacity of cooling device 900 to processheat will be realized.

In this regard, it should be noted that it is the case in manyapplications that the diameter of base 702 is often constrained to fitwithin certain predetermined form factors or dimensional requirementsand that such form factors and dimensional requirements, accordingly,have certain implications with respect to the geometric and dimensionalconfiguration of cooling device 900.

By way of example, the diametric requirements placed on base 702 mayserve to limit the overall height and width of cooling device 900 (see,e.g., FIG. 4D). In contrast however, the overall length of headerassembly 700 is generally not so rigidly constrained. Accordingly,,certain aspects of cooling device 900, such as its length for example,may desirably be adjusted to suit the requirements of a particularapplication. In the case of a TEC, for example, such a dimensionalincrease translates into a relative increase in the amount of heat thatcooling device 900 can process. As noted earlier, such heat processingmay include transmitting heat to, and/or removing heat from, one or moreof the electronic components 1000, such as laser 1002.

Moreover, various dimensions and geometric aspects of cooling device 900may be varied to achieve other thermal effects as well. By way ofexample, in the event cooling device 900 comprises a TEC, a relativelysmaller cooling device 900 with a correspondingly low load and thermalmass will permit relatively quicker changes in the temperature ofelectronic devices 1000 mounted thereto. The low thermal mass of theload of the TEC enables rapid thermal servoing and thus high-bandwidthtemperature control. In the case where electronic device 1000 comprisesa laser, this capability is particularly desirable as it lends itself tocontrol of laser performance through the vehicle of temperatureadjustments.

Turning now to consideration of the power requirements for coolingdevice 900, at least where it comprises a TEC, and electronic devices1000, it was suggested earlier herein that those devices typically relyfor their operation on a supply of electrical power. Generally, the TECmust be electrically connected with platform 800 so that power for theoperation of the TEC, transmitted from a power source (not shown) toplatform 800, can be directed to the TEC. Additionally, power issupplied to electronic devices 1000 by way of platform 800, andelectronic devices 1000 must, accordingly, be connected with one or moreof the conductive pathways 806 of platform 800.

The foregoing electrical connections and configurations may beimplemented in a variety of ways. Various aspects of exemplaryconnection schemes are illustrated in FIGS. 4A, 4B and 4E. Withreference first to FIG. 4B, the underside of submount 902 of coolingdevice 900 is connected with conductive elements 814 disposed on theunderside of first feedthrough 802, by way of connectors 816 such as,but not limited to, wire bonds. Such conductive elements 814 may beelectrically connected with selected conductive pathways 806 (see FIG.4A) and/or connector 810, that are ultimately connected with anelectrical power source (not shown).

Directing attention next to FIG. 4A, details are provided concerningvarious aspects of the electrical connection of electronic devices 1000disposed on submount 902. As noted earlier, and illustrated in FIG. 4A,some embodiments of platform 800 include one or more cutouts 811, orother geometric feature that, that permits direct connection ofelectronic devices 1000, such as laser 1002 to one or more conductivepathways 806 disposed within first feedthrough 802 of platform 800. Thisconnection may be implemented by way of connectors 816, such as bondwires, or other appropriate structures or devices. In addition to theaforementioned connection, and as illustrated in FIG. 4E, at least someembodiments of the invention further include a flex circuit 820, orsimilar device, which serves to electrically interconnect platform 800of header assembly 700 with another device, such as a printed circuitboard.

With attention now to FIGS. 4A through 4D, details are providedconcerning various operational aspects of header assembly 700. Ingeneral, power is provided to laser 1002 and/or other electricalcomponents 1000 by way of connector 810, conductive pathways 806, andconnectors 818. In response, laser 1002 emits an optical signal. Heatgenerated as a result of the operation of laser 1002, and/or otherelectronic components 1000, is continuously removed by cooling device900, which comprises a TEC in at least those cases where a laser 1002 isemployed in header assembly 700, and transferred to second feedthrough812 upon which cooling device 900 is mounted. Ultimately, secondfeedthrough 812 transfers heat received from cooling device 900 out ofheader assembly 700.

Because cooling device 900 is disposed within hermetic chamber 706, thecold junction on cooling device 900, where it comprises a TEC, does notproduce any undesirable condensation that could harm other components ordevices of header assembly 700. Moreover, the substantial elimination ofpassive heat loads on cooling device 900, coupled with the definition ofa relatively short thermal path between electronic components 1000, suchas laser 1002, and cooling device 900, further enhances the efficiencywith which heat can be removed from such electronic components and,accordingly, permits the use of relatively smaller cooling devices 900.And, as discussed earlier, the relatively small size of cooling device900 translates to a relative decrease in the power required to operatecooling device 900. Yet other operational aspects of embodiments of theinvention are considered in further detail below in the context of thediscussion of a laser control system.

While, as noted earlier in connection with the discussion of FIGS. 4Athrough 4D, certain effects may be achieved by locating cooling device900 within hermetic chamber 706, it is nevertheless desirable in somecases to locate the cooling device outside of the hermetic chamber.Aspects of an exemplary embodiment of such a configuration areillustrated in FIG. 4F, where an alternative embodiment of a headerassembly is indicated generally at 1100. As the embodiment of the headerassembly illustrated in FIG. 4F is similar in many regards to one ormore of the embodiments of the header assembly discussed elsewhereherein, the discussion of FIG. 4F will focus primarily on certainselected aspects of the header assembly 1100 illustrated there.

Similar to other embodiments, header assembly 1100 includes a base 1102having a device side 1102A and a connector side 1102B, through which aplatform 1200 passes in a substantially perpendicular orientation. Theplatform 1200 includes an inside portion 1202A and an outside portion1202B. One or more electronic devices 1300 are attached to insideportion 1202A of platform 1200 so as to be substantially enclosed withina hermetic chamber 1104 defined by a cap 1106 and base 1102. In theevent that electronic device 1300 comprises an optical device, such as alaser, cap 1106 may further comprise an optically transparent portion,or window, 1106A to permit optical signals to be transmitted from and/orreceived by one or more electronic devices 1300 disposed within hermeticchamber 1104.

With continuing reference to FIG. 4F, platform 1200 further comprises afirst feedthrough 1204, upon which electronic devices 1300 are mounted,joined to a second feedthrough 1206 that includes an inside portion1206A and an outside portion 1206B. The outside portion 1206B of secondfeedthrough 1206 is, in turn, thermally coupled with a cooling device1400. In the illustrated embodiment, cooling device 1400 comprises aTEC. However, other types of cooling devices may alternatively beemployed.

In operation, heat generated by electronic devices 1300 is transferred,generally by conduction, to second feedthrough 1206. The heat is thenremoved from feedthrough 1206 by way of cooling device 1400 which, insome embodiments, comprises a TEC. As in the case of other embodiments,a TEC may also be employed, if desired, to add heat to electronicdevices 1300.

Thus positioned and arranged, cooling device 1400 is able not only toimplement various thermal effects, such as heat removal or heataddition, with respect to electronic devices 1300 located inside oroutside hermetic chamber 1104, but also operates to process passive heatloads, which may be conductive, convective and/or radiative in nature,imposed by various components such as the structural elements of headerassembly 1500. As noted herein in the context of the discussion ofvarious other embodiments, variables such as, but not limited to, thegeometry, placement, and construction materials of platform 1200 andcooling device 1400 may be adjusted as necessary to suit therequirements of a particular application.

Further, by locating the cooling device 900 external to the hermeticchamber, additional space is available in the hermetic chamber fordevices such as laser control circuits, laser power supplies, etc.

As suggested earlier, the cooling devices constructed and operatedaccording to the invention may be usefully employed in the context of alaser control system. The laser control system includes a master controlcircuit, which may be a system that uses, for example, analog feedbackor a digital microcontroller or microprocessor using A/D and D/Acircuits The master control circuit directly controls two or, in someinstances, three outputs. These outputs include the laser outputirradiance and the TEC power (through a “power source” or TEC driver).It optionally, in the case of an EML, the master control circuit alsocontrols modulator bias. Feedback to the control system is involves two,or in some instances, three inputs. These inputs include laser launchirradiance detected by means of a monitor photodiode (MPD), or backfacet (BF) monitor and laser temperature detected by means of athermistor or another temperature sensor. The inputs can also includewavelength detected by means of a wavelength locker, using two diodes oranother suitable system. The sensors for measuring the inputs are in theheader, while the bulk of the control circuit is on an external PCB.

Because the TEC facilitates the transfer of heat from the laser, thelaser control system maintains the temperature of the laser below acritical value at which laser performance begins to degrade andreliability becomes an issue. In addition, embodiments of the lasercontrol system of the invention also enable control of the temperatureof the laser within a specified range independent of ambient temperatureconditions, so as to achieve certain ends such as wavelengthstabilization. This permits the laser to be used for a DWDM application,for example.

3. Externally Modulated Lasers Used with Modular Transceivers andTransponders

When designing a transistor header for implementation in a transceivermodule, it is desirable to limit the power consumption of the modulesuch that the power consumption is within the specification of aparticular standard for which the transceiver is designed. For example,it may be desirable to limit the power consumption of the transceiver to3.5 W or another specified value to comply with, for example, the XFPstandard. This power consumption includes the power required to operatethe active devices such as lasers and control circuitry and the powerrequired to actively pump heat away from heat generating devices in themodule. The active temperature control, using devices such as a TEC,effectively conducts heat from the active devices within the header sothe heat can be dissipated outside of the header using passive coolingdevices, such as heat sinks. To stay within the 3.5 W or other specifiedlimits, the TEC or other active cooling system and the heat sinks orother passive cooling devices must cooperate to transfer the thermalenergy from the laser, thereby actively controlling the temperature ofthe laser. The active temperature control provided by the TEC controlswavelength stability of the laser and can enable the operatingtemperature of the laser to be selected.

The ability to dissipate heat from an assembly utilizing a transistorheader via a passive cooling device is dependant on several factorsincluding the materials used in the assembly, the surface area of thematerials at various points, the temperature at which the heatgenerating components operate and the ambient temperature in which theassembly operates. The factors can be summarized by the equation:$H = {{kA}\frac{\left( {T_{o} - T_{a}} \right)}{L}}$Where H is the amount of heat transferred, k is a material constant, Ais a surface area, T_(o) is the operating temperature of the transistorheader assembly, Ta is the ambient temperature in which the transistorheader assembly is operated and L is the length of the passive coolingdevice. Thus, heat flow is dependant on the temperature differentialbetween the ambient temperature and the operating temperature. As such,if all other factors are held constant, an increase in the operatingtemperature causes a greater amount of heat to be transferred throughthe passive cooling device.

In one embodiment of the invention using an EML, although other types oflasers and active devices may be used in similar embodiments, byoperating the EML at 40° C., the passive cooling achieved by conductingthe heat generated by the laser and other devices such as the laserdriver to an external heat sink is increased as compared to operatingthe embodiment at 25° C. or 30° C. because the differential between theoperating temperature and the ambient temperature is increased. Given acertain amount of power dissipation within a header, there is a certainamount of heat that must leave the header and be dissipated into theenvironment to reach thermal equilibrium. The thermal resistance of thethermal path then determines the temperature difference between theinside of the header and the ambient temperature, according to theequation above. Assuming an ambient environment of 25° C. or so, thelaser, while operating, may get up to an idle temperature in the rangeof 45° C. to 50° C. without any active cooling because of this thermalresistance. If a TEC were used to cool the EML down nearly to theambient temperature of 25° C., the inefficiencies of the TEC wouldrequire very large amounts of power. The power required to operate theTEC can be reduced by cooling the EML from the idle temperature of 45°C. to 50° C. to only 40° C. to 45° C. Thus, the amount of activetemperature control that would otherwise be required can be reduced.Alternatively, the active temperature control may be located outside ofthe transistor header.

While this increase in operating temperature compared to an operatingtemperature of 25° C. has some effect on the wavelength of the laserbeam transmitted by the EML (as well as other operating parameters),this effect can be counteracted by varying the current supplied to theEML or by adjusting the signal to the EML driver. In an alternativeembodiment, the EML may be specifically optimized to operate efficientlyat 40° C. This optimization can be done by adjusting theelectro-absorption band-gap of the modulator when manufacturing themodulator.

Directing attention now to FIG. 6, the illustration shows an EML 2160implemented in a transistor header 2102 wherein the transistor header2102 is implemented in an optical subassembly 2100. The EML opticalsubassembly 2100 may be later installed in other components such as apluggable transceiver module or any other suitable device. The EMLoptical subassembly 2100 incorporates a transistor header 2102 with acollimating lens assembly 2104, an isolator 2106, and a receptacle 2110.

The subassembly 2100 generally comprises an outer casing 2108 forcontaining or stabilizing the other components including the transistorheader 2102, the collimating lens assembly 2104, the isolator 2106, andthe receptacle 2108. The outer casing 2108 may be constructed of anysuitable material, such as stainless steel.

In one embodiment of the invention, internal to the casing 2108 anddisposed in the transistor header 2102 is a laser diode 2160. The laserdiode 2160 may be any laser suitable for the particular application. Forexample, in a DWDM network, it may be desirable to use EMLs to takeadvantage of their narrow line width and low chirp values. Inapplications where precise wavelength control is not required, othertypes of lasers such as DFB lasers may be used. Alternatively, when thesubassembly 2100 is intended to be used as a receiver, a photodiode suchas an APD or pin diode or any other suitable diode may be used.

A collimating lens assembly 2104 is optically coupled to the laser diode2160. The collimating lens assembly 2104 may be any suitable combinationof lenses adapted to focus light from the laser diode 2160 such that thelight can be further propagated in a fiber optic network. In a receiverapplication when a photo diode is used, the collimating lens assembly2104 is adapted to focus light from the fiber optic network onto thephoto diode.

The isolator 2106 is adapted to prevent back reflection of light intothe laser diode 2160. Back-reflections are generally caused when lighttravels from a medium having a first index of refraction into a mediumwith a second, different index of refraction. Reflections back into alaser look like another cavity of the laser other than the primary, anddestabilize the amplitude and wavelength of the laser light. Certainstandards have been developed that specify acceptable amounts ofback-reflection. For example, SONET specifications require that areceiver have a back-reflection ratio no greater than −27 dB. Othertechniques can be used at the receiver to reduce optical return loss orback reflections, including a variety of index matching andanti-reflection techniques, such as a combination of fiber stubs, anglepolished fibers or stubs, anti-reflection coatings, and glass plates.

A receptacle 2110 is optically coupled to the isolator 2106. Thereceptacle is adapted to couple to other fiber-optic device in apluggable manner. In one embodiment of the invention, the receptaclecomplies with the XFP standard receptacle size for implementation in anXFP system, which is an LC fiber-optic cable receptacle. Other commonreceptacles are the SC and FC connectors.

Further disposed in the transistor header 2102 as described elsewhereabove, is a TEC cooler 2112. Also as noted above, the TEC cooler may beremoved or replaced with other types of circuits when the subassemblydesign allows for less cooling, or when there is no need for activewavelength stabilization in, for example, CWDM systems or systems thatdo not use wavelength division multiplexing.

FIG. 7 illustrates an embodiment of the invention in which an EML isincorporated into a pluggable transceiver module 2200. The opticaltransceiver of FIG. 7 conforms to the XFP standard. The transceivermodule 2200 incorporates a transmitter optical subassembly 2100 asdescribed in FIG. 6. Additionally, the transceiver module 2200 comprisesa receiver subassembly 2202 for receiving optical signals. In onepreferred embodiment of the invention, the receiver subassembly 2202comprises an avalanche photodiode for receiving optical signals. Boththe transmitter optical subassembly 2100 and the receiver subassembly2202 are disposed in a transceiver module casing 2204 so as to bepluggable into interfaces of the same standard. For example, thereceptacles 2110 and 2206 are arranged such that their spacing and sizeare appropriately configured to mate with other XFP components.

The receiver subassembly 2202 and transmitter optical subassembly 2100are electrically coupled to a module circuit board 2208. The transmitteroptical subassembly 2100 and receiver subassembly 2202 may be coupled tothe module circuit board 2208 in one embodiment, using flexible circuitboards 2210 and 2212 in an arrangement as shown in FIG. 6. The modulecircuit board 2208 may comprise various types of circuitry as necessaryfor the proper operation of the transceiver module 2200. Such circuitrymay include protective circuitry, control circuitry, power supplycircuitry and so forth. In one embodiment of the invention, the modulecircuit board 2208 includes an edge connector 2214, placed at theposterior end of the transceiver module, with contacts arranged suchthat the transceiver module 2200 is pluggable into other circuits withinthe optical fiber communication network, such as a host system.

The transceiver module may further comprise an advantageous bail release2216 useful for selectively installing and removing the transceivermodule 2200 from an optical network. The bail release is coupled to theanterior end of the transceiver module casing. This feature isespecially useful in light of the fact that pluggable standards such asthe XFP standard are intended to be hot-pluggable such that there is aneed for efficient ways of implementing components without disturbingother components in the optical network resulting in disruption ofservice by those components. The bail release shown in this example isfurther described and its advantages set forth in U.S. Pat. No.6,439,918, issued Aug. 27, 2002, which is incorporated herein byreference.

In one embodiment of the invention, the transceiver module includes amodule casing cover (not shown) coupled to the transceiver module casingto enclose the transceiver module. The case cover and the transceivermodule casing protect the transceiver module from the ingress of foreignparticles or objects.

As such, a transmitter optical subassembly utilizing lasers such as EMLsnot previously able to be used in pluggable applications, is effectivelyimplemented. Such transmitter optical subassemblies can be furtherintegrated into optical systems to create a modular optical transmissionnetwork with good bandwidth and transmission distance characteristics.Moreover, a transceiver module may be constructed that incorporates EMLsor other lasers for long-range or dense wavelength division multiplexingapplications.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in, all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A transceiver module, comprising: a transmitter optical subassemblycomprising: a header assembly that includes: a base having a first sideand a second side; and a platform attached to the base and disposed in apredetermined orientation with respect to the base, the platform havingan inside portion proximate the first side of the base and an outsideportion proximate the second side of the base, and the platformincluding a conductive pathway extending through a portion of theplatform; and an externally modulated laser (EML) supported by theplatform of the transmitter optical subassembly and in electricalcommunication with the conductive pathway of the platform; and areceiver optical subassembly.
 2. The transceiver module as recited inclaim 1, wherein the transceiver module is compatible with data rates atleast as high as about 10 Gb/s.
 3. The transceiver module as recited inclaim 1, wherein the transceiver module is substantially compliant withthe XFP MSA.
 4. The transceiver module as recited in claim 1, whereinthe transceiver module is hot-pluggable.
 5. The transceiver module asrecited in claim 1, wherein the receiver optical subassembly includes anavalanche photodiode (APD).
 6. The transceiver module as recited inclaim 1, wherein: boundaries of a predetermined temperature range aredefined by an idle temperature associated with the EML, and an ambienttemperature associated with the optical transceiver; and the EML iscompatible with EML operational temperatures relatively closer to theidle temperature than to the ambient temperature.
 7. The transceivermodule as recited in claim 1, wherein the EML is compatible with EMLoperational temperatures in a range of approximately 80 to 90 percent ofidle temperature.
 8. The transceiver module as recited in claim 1,further comprising a passive cooling device in thermal communicationwith the EML.
 9. The transceiver module as recited in claim 1, furthercomprising an active cooling device in thermal communication with theEML.
 10. The transceiver module as recited in claim 1, furthercomprising a master control circuit operably connected with the EML andconfigured to control modulator bias of the EML.
 11. A transceivermodule, comprising: a transmitter optical subassembly comprising: aheader assembly that includes: a base having a first side and a secondside; and a platform attached to the base and disposed in apredetermined orientation with respect to the base, the platform havingan inside portion proximate the first side of the base and an outsideportion proximate the second side of the base, and the platformincluding a conductive pathway extending through a portion of theplatform; and an externally modulated laser (EML) supported by theplatform of the transmitter optical subassembly and in electricalcommunication with the conductive pathway of the platform; an activecooling device arranged for thermal communication with the EML; and areceiver optical subassembly.
 12. The transceiver module as recited inclaim 11, wherein the transceiver module is compatible with data ratesat least as high as about 10 Gb/s.
 13. The transceiver module as recitedin claim 11, wherein the transceiver module is substantially compliantwith the XFP MSA.
 14. The transceiver module as recited in claim 11,wherein the transceiver module is hot-pluggable.
 15. The transceivermodule as recited in claim 11, wherein the receiver optical subassemblyincludes an avalanche photodiode (APD).
 16. The transceiver module asrecited in claim 11, wherein: boundaries of a predetermined temperaturerange are defined by an idle temperature associated with the EML, and anambient temperature associated with the optical transceiver; and the EMLis compatible with EML operational temperatures relatively closer to theidle temperature than to the ambient temperature.
 17. The transceivermodule as recited in claim 11, wherein the EML is compatible with EMLoperational temperatures in a range of approximately 80 to 90 percent ofidle temperature.
 18. The transceiver module as recited in claim 11,wherein the active cooling device comprises a thermoelectric cooler(TEC).
 19. The transceiver module as recited in claim 11, furthercomprising a master control circuit configured to control: modulatorbias of the EML; and operation of the active cooling device.
 20. Thetransceiver module as recited in claim 11, wherein the EML comprises adistributed feedback (DFB) laser.